D2D communication is a well-known and widely used component of many existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems or technologies operate in unlicensed spectrum.
Recently, D2D communications as an underlay to cellular networks have been proposed as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment.
It is suggested that such a D2D communication shares the same spectrum as the cellular system, for example by reserving some of the cellular uplink resources for D2D purposes. Allocating dedicated spectrum for D2D purposes is a less likely alternative as spectrum is a scarce resource and (dynamic) sharing between the D2D services and cellular services is more flexible and provides higher spectrum efficiency.
D2D applications include direct discovery and direct communication. In both cases, the transmitter sends D2D signals that should be directly received at least by the intended receivers. D2D should also be able to operate in multi-carrier scenarios where cellular and/or D2D is configured to operate on multiple carriers. Such carriers do not necessarily belong to a single operator and are not necessarily coordinated and synchronized.
For cellular UEs, a report maybe transmitted in the uplink to a eNB or a base station, the reporting including buffer status of queued but not yet transmitted data at layer 2. The eNB schedules uplink transmission(s) with a suitable transmission format (TF). The eNB then received the uplink transmission including the data. The eNB may further instruct the UE to perform retransmissions of the data when reception and/or detection of the previous transmission attempt fails. A similar principle is also possible in D2D where UEs provide a D2D buffer status report to the eNB and the UEs may transmit data with a TF assigned by the eNB by means of e.g. an uplink grant.
In the evolution of D2D, one suitable mode of operation comprises of splitting the control plane of D2D between a cellular path; where the eNB controls some D2D transmission resources; and a D2D path where D2D capable UEs provide control information directly to each other. In such a mode, the eNB may perform resource allocations for D2D by assigning radio resources and/or TF for D2D transmission on a direct link. If retransmission(s) in the D2D link is supported, the D2D capable UE receiver is configured to send feedback information, e.g. Acknowledgment and Non-Acknowledgement, to the D2D capable UE transmitter over the D2D direct link/path.
A problem arises when the feedback does not reach the eNB and in this case the eNB is not able to allocate the correct amount of resources. In such a scenario, since the eNB controls the allocation of resources, a mismatch may arise between expected D2D link performance and actual D2D link performance. For example, the D2D capable UE transmitter may attempt to retransmit data on resources that have not been allocated or adjusted by the eNB. This leads to an increase in transmission latency and even loss of data.